158 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



oral pinnules and the usiialh' excentric mouth, their assistance is not needed. In 

 other groups also there are certain features by which, after prolonged study, the 

 distal pinnules of any genus may be almost with certainty identified, though such 

 identification is attended with considerable risk, and owing to the greater reliance 

 which can be placed on other structures examination of the distal pinnules is 

 rarely needed. 



Pinnules and cirri. 



Though in no way related morphologically both the pinnules and the cirri are 

 linear series of cylindrical ossicles united end to end, and as such they have many 

 characters in common, and are subject to a limited extent to the same variations. 



In both the pinnules and the cirri the greater part of the possible interarticular 

 motion is localized in the basal portion, often almost entirely between the first and 

 second segments. 



When the oral and outer pinnules diifer widely the characters common to the 

 pinnules and the cirri unite the latter with the oral pinnules and the pinnules of the 

 division series, for the reason that thej' arise from the projection of the characters 

 of the calyx and division series upon them, and the projection of these characters 

 does not reach the distal pinnules. 



The proximal pinnules, like the proximal cirri, are typically longer than the 

 others and possess tactile functions (part 1, figs. 310, 311, p. 269). In both cases 

 it is to be observed that elongation and added sensitiveness is characteristic of the 

 organs situated nearest to the central capsule of the dorsal nervous system. 



The function of the cirri is to serve as prehensile organs. The same function 

 is assumed by the grooveless pinnules distributed at intervals along the arms in 

 ComatuleUa hrachiolata (plate 1, fig. 1332), which are capable of being coiled 

 dorsally to an unusual extent. 



In certain types, particularly in the species of Pronietra, Golobometra^ and 

 Oligoinstra, there is a marked correlation between the number of ossicles in the 

 cirri and in the oral pinnules. 



In other tj'pes, as in certain species of Heterometra, the development of the 

 dorsal ridge on the oral pinnules is correlated with the amount of carination of 

 the distal cirrus segments. 



On the pinnulars, as on the cirrals, the distal dorsal edge is often produced 

 into a frill of long spines; in both sets of structures these spines are never found 

 on the first two segments, but begin on the third and gradually increase in promi- 

 nence outwardly; in both also this projection from the dorsal surface is at first 

 limited to the distal edge, but distally gradually moves inward. 



The growth of the pinnules and of the cirri takes place in exactly the same 

 manner, and in both the terminal ossicle is well formed before the one or more 

 preceding have acquired their full length. 



The cirri end in a strong spine, while one or more strong spines are developed 

 on the dorsal surface of the last pinnular. On the antepenultimate segment in 

 both structures one or more spines are developed, and spines of diminishing size 

 may occur for some distance toward the base on the preceding ossicles. 



